Recently, studies on a nano-structural material have been conducted in various fields. Particularly in the field of powder sintering, a technology has been studied and developed that is directed to produce a sintered compact (for e.g., a nano-phase material based on metals or ceramics, a nano-composite material and the like) by using a nano-sized powder of a starting material. A sintering process typically takes a long time with a prior art sintering process that simply provides heating and pressing, in which even if a nano-sized powder of a starting material is advantageously used for sintering, grain growth is likely to occur in the course of sintering and indeed, the observation on a grain diameter of a thus sintered compact shows that the crystalline grain has grown significantly as compared to the original nano size of the starting material. In the above-described circumstances, there has been a need for sintering technology to control the minute structure of the grain or to control grain growth, which requires essentially that the sintered process be carried out in a short time.
The inventors of the present invention have devoted themselves for many years to the development of, what is called, the pulse energization and pressure sintering process, such as the spark plasma sintering or the plasma activated sintering, as well as a sintering apparatus and system with which the same sintering process can be carried out, and they have found that the pulse energization and pressure sintering process is extremely useful for sintering with use of the nano-sized powder of a material (herein, the term, nano-sized powder of a material, refers to a powder having a powder size in itself defined by the nano order and a powder having the powder size defined by the micron order but comprising a crystalline structure defined by the nano order). However, they have also found that suppressing the grain growth simply by using the conventionally known pulse energization and pressure sintering apparatus would be insufficient to produce a highly purified sintered compact with ultra high precision and high function and also having a structure defined by a nano-sized crystal grain diameter by sintering a nano-sized powder of a material containing no impurities. This is due to a drawback explained by a known fact that the nano-sized powder has a larger specific surface area compared with an ordinary type of powder defined by the micron order and also has a high activity in the grain surface that could form an oxide film easily, when exposed to the atmosphere, which oxide film could be deposited on the grain boundary or dispersed internally during synthesizing of the material, thus inhibiting a desired property from being obtained.
List of Patent Documents
[Patent Document 1]                Japanese Patent Laid-open Publication No. 2000-345208        